1. Field of the Invention
The present invention relates to a novel rare earth gallate oxide-ion conductor, having a perovskite structure. The oxide-ion conductor of the present invention exhibits very high oxide-ion conductivity or oxide-ion mixed conductivity without being substantially affected by oxygen partial pressure, and can effectively be incorporated in an electrolyte of a fuel cell, an air electrode of a fuel cell, a gas sensor such as oxygen sensor, an oxygen separating film such as electrochemical oxygen pump, a gas separator membrane, and so forth.
2. Discussion of the Background
An oxide-ion conductor owes its electrical conductivity mainly to the mobility of oxide ions (O2−), without substantially relying on conductivity by electrons. In general, such an oxide-ion conductor is made of a metal oxide that is doped with another metal, so as to generate O2− vacancies. Attempts have been made to put such oxide-ion conductors to use in various types of materials, such as electrolytes of solid oxide (solid electrolyte) fuel cells (SOFC), gas sensors, e.g., oxygen sensors, and oxygen separator membranes of electrochemical oxygen pumps.
A typical example of such oxide-ion conductors are cubic fluorite type solid-solutions referred to as “stabilized zirconia” composed of zirconium oxide (ZrO2) containing a small quantity of dissolved divalent or trivalent metal oxide, such as CaO, MgO, Y2O3, Gd2O3 or the like. The stabilized zirconia excels in heat resistance, and has conductivity that is predominantly provided by oxide ions over the entire range of oxygen partial pressure, i.e., from a pure oxygen atmosphere to a hydrogen atmosphere. Thus, the stabilized zirconia is less liable to exhibit reduction in the ion transference number (the ratio of conductivity given by the oxide ions to the overall conductivity), even under reduced oxygen partial pressure.
Zirconia oxygen sensors made of the stabilized zirconia are used for various purposes, such as control of industrial processes including steel making, air-fuel ratio control of automotive engines, and so forth. The stabilized zirconia is also used as the material of a solid oxide fuel cell (SOFC) which is being developed and which operates at temperatures around 1000° C. It is to be noted, however, that the oxide-ion conductivity of stabilized zirconia is not so high, and tends to cause insufficiency of electrical conductivity when the temperature is lowered. For instance, the ion conductivity of a Y2O3 stabilized zirconia exhibits an ion conductivity which is as high as 10−1 S/cm at 1000° C. but is reduced to 10−4 S/cm when the temperature is lowered to 500° C. This stabilized zirconia, therefore, is usable only at high temperatures not lower than 800° C.
Fluorite type compounds exhibits a very high oxide-ion conductivity exceeding that of stabilized zirconia. An example of such a fluorite type compound is a Bi2O3-type oxide composed of Bi2O3 containing dissolved Y2O3 in the form of a solid solution. Such a fluorite type compound, however, has a low melting point of 850° C. or less, thus exhibiting inferior resistance to heat, although it exhibits very high levels of ion conductivity. In addition, the fluorite type compounds is not resistant to a reducing atmosphere. More specifically, when the oxygen partial pressure are lowered, n-type electron-based electrical conductivity prevails due to a change in the oxidation state of Bi3+ to Bi2+. A further reduction in the oxygen partial pressure to a level approximating a pure hydrogen atmosphere causes the compound to be reduced to the metal. The fluorite type compounds, therefore, cannot be used as a material for fuel cells.
Another kind of known fluorite type oxide-ion conductor is a ThO2 type oxide. This oxide exhibits oxide-ion conductivity much smaller than that of stabilized zirconia. In addition, electron-based electrical conduction becomes dominant so as to markedly lower the ion transference number, particularly under low oxygen partial pressures. A CeO2 type oxide, although it exhibits oxide-ion conductivity exceeding that of stabilized zirconia, permits n-type electron-based electrical conduction to prevail due to a change in the oxidation state of Ce4+ to Ce3+ when the oxygen partial pressure is reduced to 10−12  atm or less. Consequently, reduction of the ion transference number is also unavoidable with this type of compound.
Oxide-ion type conductors are also known that have crystalline structures other than the fluorite structure. Examples of such oxide-ion type conductors are PbWO4, LaAlO3, CaTiO3 and so forth. These conductors, however, do not have high oxide-ion conductivity and, under low oxygen partial pressure, allow semi-conduction to appear so that electron-based electrical conduction prevails, resulting in a low ion transference number.